Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
FIGS. 1 and 2 illustrate a fixing device 1R installed in such image forming apparatuses, which includes a fixing roller 101 and an opposed pressing roller 102 that apply heat and pressure to a recording medium P bearing a toner image. For example, the pressing roller 102 is pressed against the fixing roller 101 heated by a heater 106 disposed inside the fixing roller 101 to form a fixing nip N therebetween through which the recording medium P bearing the toner image is conveyed. As the fixing roller 101 and the pressing roller 102 rotate and convey the recording medium P through the fixing nip N in a recording medium conveyance direction F, the fixing roller 101 and the pressing roller 102 apply heat and pressure to the recording medium P, melting and fixing the toner image on the recording medium P.
Pressure applied between the pressing roller 102 and the fixing roller 101 is adjustable according to the type of the recording medium P in order to accommodate envelopes produced by bonding a folded sheet, which are susceptible to warping and creasing when passing through the fixing nip N. For example, as shown in FIG. 1, when plain paper is used as a recording medium P, the pressing roller 102 presses against the fixing roller 101 with enhanced pressure, thus forming a longer fixing nip N in the recording medium conveyance direction F, to apply sufficient heat and pressure to the plain paper. Conversely, as shown in FIG. 2, when an envelope is used as a recording medium P, the pressing roller 102 presses against the fixing roller 101 with reduced pressure, thus forming a shorter fixing nip N in the recording medium conveyance direction F to prevent creasing of the envelope.
As a mechanism for changing the pressure between the pressing roller 102 and the fixing roller 101, a configuration may be employed in which a lever 115 moves a pressing frame 111 mounting the pressing roller 102 with respect to a fixing frame 110 mounting the fixing roller 101 by leverage, as shown in FIGS. 3 and 4. For example, a spring presses the pressing frame 111 against the fixing frame 110 to press the pressing roller 102 against the fixing roller 101. As the lever 115 pivotably attached to the pressing frame 111 swings in a direction A1 (clockwise) about a shaft 116 from an enhanced pressure position shown in FIG. 3 where the pressing roller 102 presses against the fixing roller 101 with enhanced pressure to a reduced pressure position shown in FIG. 4 where the pressing roller 102 presses against the fixing roller 101 with reduced pressure, one end of the lever 115, that is, a point of load, contacts the fixing frame 110 as shown in FIG. 4, thereby swinging the pressing frame 111 in a direction B1 about a shaft 103. Accordingly, the pressing roller 102 moves away from the fixing roller 101, producing the shorter fixing nip N through which an envelope P2 is conveyed. Conversely, as the lever 115 swings in a direction A2 (counterclockwise) from the reduced pressure position shown in FIG. 4 to the enhanced pressure position shown in FIG. 3, the one end of the lever 115 no longer presses against the fixing frame 110, thereby swinging the pressing frame 111 in a direction B2 and recovering the longer fixing nip N through which plain paper P1 is conveyed.
However, the configuration shown in FIGS. 3 and 4 has a drawback in that the single lever 115 may restrict movement of the pressing roller 102 apart from the fixing roller 101 or require a substantial force to move the pressing roller 102 apart from the fixing roller 101 farther. Additionally, a resilient bias exerted by the spring to press the pressing frame 111 against the fixing frame 110 may accelerate swinging of the lever 115 in the direction B2. Accordingly, when the lever 115 swings in the direction B2 from the reduced pressure position shown in FIG. 4 to the enhanced pressure position shown in FIG. 3, a substantial mechanical shock is generated and transmitted to the components connected to the lever 115, producing sound and degrading precise formation of the fixing nip N.
To address the above-described drawbacks of the fixing device 1R, it is possible to include a cam that engages and disengages a lever as the lever slides over the outer circumferential edge face of the cam, thus eliminating a strike of the lever against the cam that may generate noise. However, this configuration may not minimize mechanical shock caused by the lever.
Alternatively, a movable cushion or sponge that contacts the lever to absorb mechanical shock therefrom can be employed. However, an expensive solenoid may be required to separate the cushion from the lever, resulting in increased manufacturing costs.